DR. CARPENTER IN AMERICA. 7 
Clearly the two-inch objective, to be a good one, must stand the 
E eyepiece at least, for, we imagine, “resolving” means resolving 
satisfactorily with the production of a good image in every respect. 
Now the one inch produces the amplification of a quarter, when 
used with a D eyepiece, and yet we are told that the inch with D 
eyepiece makes a bad quarter, while the two inch should stand the 
B or even higher ocular. We fail to see the logic of the situation, 
and on the other hand will be willing to travel a few miles to see 
the Podura scale satisfactorily resolved with a ow angle two-inch, 
with whatever eyepiece the manipulator may choose to employ. 
Furthermore, in the quotation italicised, we find foreshadowed 
what aperture Dr. Carpenter would have given to a two-inch objec- 
tive ; for this purpose we photographed the largest scale upon our 
test slide and counted the markings. The scale measured ;j5th 
of an inch in breadth, and possessed 40 rows of markings across it, 
so that these lines were really ;5455 of an inch apart; another 
scale was measured , oth of an inch, and had 35 rows of markings, 
or containing lines ;;455 of an inch apart ;—what does /heory say 
should be the aperture which will resolve these lines? Answer: 
207 N.A., Or 24° in air. This ts the theoretical minimum angle the 
two-inch must have to resolve this scale, and the excess above this, 
actually necessary, depends upon the perfection of the corrections. 
This brings us to another point. The excess of aperture neces- 
sary to do the ¢heoretical work of the lower angle, depends upon 
the excellence of the corrections. Now it is well known that the 
cheap English glasses of low angle are without exception not so 
carefully finished as the more expensive lenses ; they are fair work- 
ing glasses, no doubt, and not to be discouraged for amateurs’ use ; 
we must, therefore, take exception to the paragraph which pits the 
widest angle American 5‘, against our cheap ones. Dr. Carpenter 
tells us that for £10 or £12 a good set of English objectives up to 
a jth of an inch may be obtained. What is the experience of our 
readers? Crouch’s list up to the ;4,th dry adds up to £29 4s. od. ; 
Swift’s list to the th adds up to £32 14s. od.; and most others 
in like proportion for their first series. It is true that these opticians, 
and others like Mr. Collins, produce a second series which adds up 
to #12 15s. 6d., or even a third series, like Mr. Wray, which up to 
the one-sixth adds up to £10 18s. od. ; but they have these lenses 
in America also: Collins’ half-inch of 45° is £2 5s. od.; the 
Bausch and Lomb half inch of 42° is £1 16s. od. ; Collins’ half-inch 
of 40° being £1 5s. od. 
Tolles ;4,ths of 140° costs £13; and we venture to say it would 
cost quite as much if made in this country. The Bausch and Lomb 
zio of 55° costs £2 55. od. ; while Powell and Lealands of 80° 3ths 
costs £5 5s. od., their half-inch of 40° £4 4s. od., and the 
goth £12 12s. od. 
